1. Field of the Invention
The present invention generally relates to a power output apparatus and a method of controlling the same. More specifically, the invention pertains to a power output apparatus for efficiently transmitting or outputting a power from an engine to a drive shaft and a method of controlling such a power output apparatus.
2. Description of the Related Art
In proposed power output apparatuses mounted on a vehicle, an output shaft of an engine is electromagnetically connected to a drive shaft linked with a rotor of a motor via an electromagnetic coupling, so that power of the engine is transmitted to the drive shaft (as disclosed in, for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 53-133814). When the revolving speed of the motor, which starts driving the vehicle, reaches a predetermined level, the proposed power output apparatus supplies an exciting current to the electromagnetic coupling in order to crank the engine, and subsequently carries out fuel injection into the engine as well as spark ignition, thereby starting the engine and enabling the engine to supply power. When the vehicle speed is lowered and the revolving speed of the motor decreases to or below the predetermined level, on the other hand, the power output apparatus stops the supply of exciting current to the electromagnetic coupling as well as fuel injection into the engine and spark ignition, thereby terminating operation of the engine.
In the known power output apparatus described above, the torque output to the drive shaft is significantly varied at the time of starting and stopping the engine. This results in a rough ride. At the time of starting the engine, the torque output from the motor is used to crank the engine, and the torque output to the drive shaft is decreased by the amount required for cranking. At the time of stopping the engine, the supply of exciting current is stopped while the power from the engine is transmitted to the drive shaft via the electromagnetic coupling, and the torque output to the drive shaft is decreased by the amount of power transmitted from the engine. Such a fall in output torque occurs unexpectedly since the driver does not determine the time of starting or stopping the engine. Compared with the expected variation, the unexpected variation in output torque to the drive shaft gives a greater shock to the driver, thereby resulting in a rough drive.